Method and apparatus for television communication



455-605 AU 233 EX FIPBlOb xa 2,330,682

Sept. 28, 1943. s. L CLOTHIER El AL 2,330,682

. IETHOD AND APPARATUS FOR TELEVISION COMMUNICATION Original Filed March15, 1938 3 Sheets-Sheet 1 .ATT NEY Sept. 28, 1943. s. L. CLOTHIER ETAL2,330,682

IETHQD AND APPARATUS FOR TELEVISION COMMUNICATION Original Filed March15, 1938 3 Sheets-Sheet 2 m I I I I I W sept- 23, 1943- s. L. CLOTHIERETAL 2,330,682

ETHOD AND APPARATUS FOR TELEVISION COMMUNICATION Original Filed larch15, 1938 3 Sheets-Sheet 5 Ham/daw n amp Patented Sept. 28, 1943 METHODAND APPARATUS FOR TELEVISION COMMUNICATION Stewart 1. crooner, lunch,and Harold c. Hog- Iapicwood, N. 1.

Original application March 15, 1938, Serial No.

195,939, now Patent No. 2,288,523, dated December 80, 1941. Divided andthis application December 30, 1941, Serial N0. 424,9

2 Claims. (Cl. 178-72) for television reception and cathode-ray tubesfor television transmission.

In television communication employing at the receiving station acathode-ray tube having a fluorescent screen which is scanned by a rayofelectrons, it has been proposed to make the construction and thevarious electrical conditions such that the fluorescent image besumciently brilliant for projection onto a larger, external screen. Ithas been found, however, that on account of the characteristics of thefluorescent screen, these tubes have definite limitations as to thebrilliancy and sharpness of the image which can be produced forprojection. For example, when operating at the relatively highintensities required, the fluorescent materials used heretofore nolonger have a linear modulation characteristic. Furthermore, some screenmaterials which are purely fluorescent at moderate intensities becomephosphorescent and even incandescent at the higher intensities ofelectron bombardment. These effects or characteristic result inpronounced blurring of the image, for the reason that at any elementalarea of the screen, the

period of time elapsing during excitation of the fluorescent materialand decay to the point of extinction or invisibility, is substantiallygreater than the frame or fleld scanning period which is necessary toavoid flicker.

Development work along this line has accordingly centered on aconstruction for the screen whereby a more brilliant image can beobtained. One such construction comprises a thin sheet oi metal which ismaintained at red heat by current from an external source. In operation,as the ray of electrons is deflected to cause the screen to be scanned,any particular point is raised from red heat to higher values up tointense incandescence, depending upon the ray intensity. However, inthese screens the period of decay is far in excess of that allowable ifany particular point is to return from relatively intense incandescenceto red heat within the frame or fleld period necessary to avoid flicker.Furthermore.

on account of the relatively high coefiicient of heat-conductivity ofthese screens, th heat of bright points flows in all directions toequalize the temperature, thereby causing adjacent points to becomebrighter than perhaps they should be, for good detail.

Another such construction is disclosed in Patout No. 2,098,000, issuedNovember 2, 1937, to

Philo T. l 'arnsworth et a1., and comprises a screen having a surface ofrefractory textile material similar to that in a common gas mantle, andwhich is capable of being raised to incandescence by the impact of theelectron ray deflected to scan this surface, the degree of incandescenceto which any elemental area is raised being in proportion to theintensity of the electron ray di rected at such elemental area at theinstant. Screens of this construction have a relatively low coefficientof heat-conductivity, so that during any frame or field period, a brightpoint of the image does not expand and include adjacent points. In thesescreens, however, return of any particular point from incandescence tored heat or lower, is largely by radiation, which fact again gives riseto blurring because of time lag. That is, the decay period issubstantially greater than that which the frame or fleld period wouldhave to be if there is to be no flicker.

Still another construction for such a screen is disclosed in Patent No.2,097,994, issued November 2, 1937, to Harry B. Bamford, and comprisesan lncandescible screen composed of a plurality of closely mountedhelices of very fine tungsten wire having their axes parallel, andnormal to the plane of the screen. In operation, the impact of the rayof electrons on the screen raises the elemental areas thereof toincandescence at the P t Of t the degree of incandescenceat any onepoint being proportional to the intensityof the electron ray directed atsuch point at the instant. In these screens, however, there is also thedetrimental time lag, as in the others.

In order to avoid flicker, the frame frequency should be at leastsixteen, and is preferably at least twenty-four. This means that inorder to realize the desired operating action in screens of thecharacter referred to, the decay period, whether it is one of radiationor one of conduction, must be no greater than one twen y-fourth of asecond for good detail. Where interlaced scanning is employed, the fleldfrequency may be as high as sixty, in which case the decay period mustbe no greater than one-slxtieth of a second if any degree of detail isto be obtained. Even at a frame frequency of sixteen. there is blurringin the prior screens referred to on account of the time lag, and at aframe frequency of twentyfour, or at the high fleld frequency of sixty,these screens, if useful at all, would only b so for a still picture.Furthermore, in the screen construction proposed heretofore. selectionof screen materials has necessarily been limited to those having aperiod of decay or a time lag suiiiciently low for a given frame orfield frequency to avoid materials having a decay period many timestheframe or field frequency, has not been possible.

With the foregoing in mind, an object of our invention resides in theprovision of an improved method and apparatus for televisioncommunication whereby it is possible, in a cathode-ray projection tubeof the character referred to, to employ materials. such asphosphorescent or incandescible materials, having a time lag or decayperiod many times any desired frame or field period.

Another object of our mvention resides in the provision of an improvedmethod and apparatus for television communication whereby there isavailable at the receiver a sharply defined light source, modulated inaccordance with the light intensity of the corresponding element of astill or moving image to be reproduced, and which has a substantiallyhigher intensity than it is possible to obtain with the prior methodsand constructions.

. Another object of our invention resides in the provision of animproved construction of cathode-ray tube for television transmissionwhich has advantages over those proposed heretofore in the way ofgreater efllciency and better operating action.

Other objects and advantages will hereinafter appear.

For the P rp se of illustrating our invention, an embodiment thereof isshown in the drawings, wherein Figure 1 is a simplified, diagrammaticview, partly in section, of a television receiving system constructedand operating in accordance with our invention;

Fig. 2 is an elevational view partly in section, the section being takenon the line 2-2 in Fig. 1;

Fig. 8 is a view similar to Fig. 2, showing a modification;

Fig. 4 is a simplified, diagrammatic view, partly in section, of atelevision transmitting system constructed and operating in accordancewith our invention;

Fig. 5 is an enlarged, detail,- elevational view, takenonthelineHinFig.4;

Fig. 6 is an enlarged sectional view, taien on the line 8-4 in Pig. 5;

Fig.7isaviewsimiiartoFig.4,showinga modification;

Fig.8isaviewsimilartoFig.1,showinga modification;

Fig. 9 is a detail, elevational view, taken on the line 0-! in Fig. 8;

Fig. 10 is an elevational view partly in section, the section beingtaken on the line ll-ll in Fig. 8; and

Fig. 11 is a schematic diagram illustrative of the operating action inFig. 8.

' In Fig. 1 of the drawings, the reference numeral il designates acathode-ray tube provided with an electron gun ll of any suitable,conventional construction, for developing a ray ii of electrons directedat the eflective surface of the screenstructurewhichisintheformofadmmi3, rotatably supported in the .tube. For this purpose, thedrum llisfixed on a spindle ll having a bearing at each end thereof in the wallof the tube, as shown. If desired, jewels may be used for thesebearings. The effective cylindrical surassaeaa able fluorescent,phosphorescent, or incandescible material.

The drum it is rotated in the clockwise direc- 7 tion and at a uniformrate by a motor It, the

' tation of the drum it. However, the ray I! may be deflectedelectromagneticaily, if desired, and a second set of deflecting plates25 or other defiection means may be employed for vertical posiidoning ofthe ray.

In operation, deflection of the ray I: in the one dimension effects linescanning, and movement of the screen surface in the direction transverseto this dimension eilects frame scanning. With picture signals appliedto the control electrode or grid is of the electron gun to modulate theray intensity in accordance with the lights and shadows of therespective elemental areas of the image at the transmitter, a like andbrilliant image is produced at 20 on the screen surface. This image orseries of images of course moves with the screen surface. The movingimages are projected by a lens system 21 to a large external screen 23,the beam of light producing the large images being first reflected fromthe mirror drum 22, which eflectively arrests the vertical motion of thelarge image.

Assuming that the frame frequency is to be twenty-four, the drum 22 canhave twenty-four mirrors and be rotated at a constant rate of sixtyrevolutions per minute, so that twenty-four mirrors pass a given pointat the periphery in one second. The diameter of the screen drum II insuchcasewillbesuchthat withthisdrumbeing rotated at a constant rate ofsixty revolutions per minute, the screen surface may be composed oftwenty-four frame areas which are scanned by the ray i2 and presented tothe lens system 20 in succession and at the rate of twenty-four'persecond. From this it will be seen that repeated scanning of any oneelementary line of the screen structure takes place only after anelapsed period of one second, which is far in excess of the time lag ordecay period which the screen material might have. In our improvedmethod and apparatus, therefore, there is always ample time for thescreen material to decay from intense brightness to extinction before itmoves around to be scanned again. If interlaced scanning is to be used,and the field frequency is to be sixty, with each of the drums beingrotated at a constant rate of sixty revolutions per minute as before,the mirror drum 22 could have sixty mirrors and the diameter of thescreen drum It could be such that the screen surface could be composedof sixty frame areas which would be scanned by the ray l2 and presentedto the lens system 20 in succession and at the rate of sixty persecond.Otherfiguresandspeedscanbe used to obtain these same or other frame orfield speeds.

The diameter of the screen-drum I3 is not critical. However, itsperipheral speed, the frame frequency and the image size on thescreen-drum surface are directly related. For example, having thedesired size of the images at the screendrum known, itis possible to fixthe peripheral face of the drum may be composed of any suit- 76 speed ata value determined by the product of the frame frequency and the heightof one screen-drum image.

The operating action may be analyzed in the following manner. with boththe screen-drum It and the mirror or frame-drum f2 stationary. theresult on the large screen 2! will be only a single, stationary.horizontal line. With the screen-drum I3 rotating and tie mirror-drumstationary, the result is a series of images moving vertically downwardon the large screen 23. and which are visible only as a blur. However,with the mirror-drum rotating at the proper speed and in the rightdirection, the vertical movement of the image serie will be immobilizedto cause the moving images to stand still on the screen 23 and theaction of the subject to be reproduced.

In normal operation, if the screen-drum is run more slowly, the picturewill remain on the projection screen, but it will be collapsed inheight. If the screen-drum is rotated too fast. the picturewill bestretched out in height, but it will remain fixed on the projectionscreen.

The mirror drum 2! may be driven by a separate motor 24, or the samemotor may be used for driving bot-h drums II and 12.

As shown in Fig. 3, in lieu of the magnetic flux coupling between themotor II and the spindle I4, one end of the latter may pass through thewall of the evacuated tube through a grease seal, at which point thereis a conical section 26 which is held seated in the srease bearing bythe atmospheric pressure. The use of such driving means is made possibleby the slow speeds of rotation required.

From the foregoing it will be seen that in our improved method andapparatus, it is possible to use, for the screen, materials such asphosphorescent or incandescible materials which, although moredesirable, could not be used in the prior methods and constructions onaccount of their relatively high time lag or decay period.

In our improved construction, furthermore.

the greatest possible efliciency is obtained because of the fact thatthe image is taken from the same side of the screen surface which isscanned by the ray I2. Also, on account of the arrangement and operatingaction, there is no keystone and no variation in focus of the ray on thescreen surface.

In cases where it is desired to use certain types of interlacedscanning, it may be desirable to cause a small amount of intermittentdeflection ofthe ray I2 in a direction at right angles to theline-scanning dimension. in which case a second set of electrostaticplates or electromagnetic deflection coils may be used for thisdeflectlon.

In the transmitting system shown in Fig. 4, except for the following theconstruction and action is the same as in Figs. 1 and 2. The drum Ito,corresponding to the drum ii in Fig. 1, is provided with a mosaic,photosensitive surface. A

' light image of the object 21, after reflection from the mirror-drum22a, is proiected by the lens system 2 la onto the photosensitivesurface of the drum l3a. .A collector electrode, which may be in theform of a metal plate as provided with a slot 29 as shown in Figs. and6, is supported close to the drum surface at the region thereofbombarded by the electrons of the ray lie, the arrangement and size ofthe slot 2! being such that during normal operation the electrons of theray can pass freely through the electrode 28 to the drum surface,

With the drum "a rotating in the counterclockwise direction at thedesired uniform rate, and with the mirror-drum Ila rotating at theproper speed and in the right direction accordlog to the same principlein Fig. 1, there will be produced on the surface of the drum Its asuccession of complete, electron images of the object 21, each comprisedof photoelectric charges corresponding respectively to the lights andshadows at the corresponding elemental areas of the object 21. Thesephotoelectric charges would, if desired, remain on the photoelectricsurface for an appreciable time, which might be a period greater thanthat for one revolution of the drum Ila. There is therefore no loss inemciency while any part of the drum surface rotates from lightimageposition to the position at the slot 2| of the collector electrode. Aseach elemental line at of the photoelectric surface of drum Ila movesacross the slot fl, It is scanned by the ray I In which is beingdeflected at line-scanning frequency by the plates "a. Electrons ofsecondary emission, represented by the arrows 3 I, are thereby released.and in intensity corresponding to the respective photoelectric chargesalong the line 30 being scanned at the instant. On account of the closeproximity of the electrode II to the drum surface, an appreciablepercentage of the secondary emission 3| may be collected by thiselectrode and fed by the connection 32 to an amplifier and transmittercircuit, as shown. The current in the return circuit of the electrode 32will consist of a certain direct-current component modulated by thevideo signals. The elsetrade 28 need be only slightly positive withrespect to the adjacent photosensitive surface, in order to collect thedesired secondary electrons. Since the electron path is relativelyshort, the

output impedance of the collector electrode 28 is low as compared tosuch impedance in the various constructions and methods proposedheretofore. This is advantageous where a wide band of frequencies is tobe used, and results in a better signal-to-noise ratio.

After any line 30 passes beyond the collector electrode 2|, there maystill be some electron charges remaining on the mosaic surface. For thepurpose of removing such remaining charges. an erasing electrode 33 isemployed. This electrode is supported close to the mosaic surface, andmay be. for example. at about one hundred volts positive with respect tosuch surface. The mosaic surface, therefore, proceeds from the erasingelectrode 33 to the re-exposure position in an uncharged and uniformlylight-sensitive condition. Shading efiecm are thus eliminated.

In the transmittingsystem shown in Fig. 7, the object is inthe form of amoving picture film it moved at a uniform rate in front of a mask I!provided with a slit aperture 88. The rate of linear movement of thefilm may be the same as the peripheral speed of the drum Isa providedthe lens III: is arranged to give one to one image size, and this rateis determined by the desired frame frequency, as will be wellunderstood. In operation, as the film moves at constant speed past theslit aperture 3|. an image of this slit is projected onto the mosaic,photoelectric surface of the drum Ila, producing photoelectric chargethereon in accordance with the light and shade conditions along therespective linear element of the him. The operating MtiOl, otherwise, isthe same as in Figs. 4, 5

In Figs. 4 and I, the ray may be positioned with respect to the slot 29by the plates Ila.

In the receiving system shown in Fig. 8, there is no mirror-drum as inFig. I, and a difference in the construction shown in Fig. 8 resides inthe fact that the screen-drum ilb is rotated intermittently by the motorlib through a Geneva gear mechanism 31 of any suitable, conventionalconstruction. Also, in Fig. 8 a shutter SI of a conventionalconstruction is used, and is disposed between the lens system lib andthe external screen 23b. Both sets of deflecting platesareusedinlig.8,sothattherayflbis deflected simultaneously at the line-scanningfrequency and at the frame frequency to scan a frame area of the drumsurface.

In operation, while the screen-drum lib remains stationary, a completepicture is scanned onto one frame area of its surface. At the end of,one complete picture-scan, the screen-drum is rapidly moved, in thedirection indicated by the arrow, a selected distance such that thescanned picture is in the field of the projection lens. During thismovement of the screen-drum, the shutter 38 covers the projectionsurface of the lens. At the end of the intermittent movement, the drumlab is again stationary, and the shutter will have reached a position topermit passage of the beam to produce an enlarged image of the pictureon the external, viewing screen 2312. This image is projected for aperiod during which a second picture is being scanned onto a differentand succeeding frame area of the stationary screen-drum.

If desirable, to eliminate flicker, the shutter 38 may be provided witha second blade, as shown in Fig. 9, and operated as in a motion pictureprojector, to quickly cover and uncover the picture while it remainsmotionless.

when the second picture has been completed, the drum llb is again movedand the above described action is repeated.

If 24 pictures are to be produced each second, the design of the Genevagear mechanism ll could be such as to cause the drum llb to remainstationary for about 95 of a second, and to be moved to its new positionduring the next M of a second, making a total time cycle of ,5 of asecond. Such operating action is illustrated in Fig. ll.

a,sso,sse

It will be understood that the shutter 38 may llizbdriven, throughsuitable gearing, by the motor It is of course preferable to have thescreen drum constructed with a minimum of weight and low movement ofinertia to facilitate its intermittent, motion without excessive drivingpower requirements.

In Fig. 7 the intermittent drive shown in Fig. 10 might be used, inwhich case a standard motion picture projector, with intermittent filmmovement and associated shutter, would be employed, and the twointermittent movements would be synchronized so that the drum and filmwould be stationary during the-same periods. Likewise in Fig. 4 themirror-drum could be omitted, and an intermittent and shutter employedas in Fi 8.

It will be understood that various modifications, within the conceptionof those skilled in the art, are possible without departing from thespirit of our invention or the scope of the claims.

This application is a division of our prior application, Serial No.195,939, filed March 15, 1938, now Patent No. 2,268,523, December 30,1941.

We claim as our invention:

1. In a cathode-ray device for television transmission, a tube, screenstructure in the form of a drum rotatably supported in said tube andhaving a photosensitive operating surface, means for developing a ray ofelectrons directed at said surface, means for deflecting said ray, anelectrode supported in proximity to said surface at the region thereofbombarded by the electrons of said ray, and a connection from saidelectrode to a point exterior of said tube.

2. In a cathode-ray device for television transmission, a tube,photosensitive screen structure in the form of a drum rotatablysupported in said tube, means for developing a ray of electrons directedat the eflective surface of said screen structuremeans for deflectingsaid ray. means for imparting rotary movement to said drum to cause saidsurface to be scanned by said ray, and means including a mirror drum forprojecting a series of images of the view for transmission on saidmoving screen surface.

STEWART L. CLOTHIER. HAROLD C. HOGENCAMP.

